The present invention relates to a wireless transmission device, a wireless reception device, a wireless transmission program, a wireless reception program, and a wireless communication system. The present invention may be applied, for example, to a wireless communication device for intermittent reception.
For example, a communication device (which will also be referred to as “transmission device” or “reception device” below) included in a sensor network is configured to perform power-saving communication. Typical methods for power-saving communication include intermittently operating communication devices. For example, coordinated sampled listening (CSL) is a communication standard that adopts this intermittent operating method for communication devices (see IEEE802.15.4e).
A CSL sampled listening period (CSLPeriod), a maximum CSL sampled listening period (CSLMaxPeriod), active time (wakeup reception time) in which wakeup frames can be intermittently received, and the like are set for each of communication devices that adopt the CSL.
First of all, a transmission device S that attempts to transmit data executes an operation of transmitting data in an asynchronous transmission mode. The following is the asynchronous transmission mode. The transmission device S successively transmits wakeup frames to a reception device R in the maximum CSL sampled listening period. Rendezvous time (RZTime) is written in the wakeup frame, the rendezvous time (RZTime) indicating the remaining time from data transmission. Once the transmission device S finishes transmitting wakeup frames in one maximum CSL sampled listening period, the transmission device S subsequently transmits a data frame to the reception device R.
The reception device R stays in a listening state only for the active time within the one CSL sampled listening period, and does not perform any reception operation for the other time. Once the reception device R receives a wakeup frame from the transmission device S in the listening state, the reception device R acquires the rendezvous time in the frame. The reception device R is activated (listening state) after sleeping until just before the rendezvous time, and receives a data frame from the transmission device S.
The reception device R replies a reception response signal (ACK) to the transmission device S upon receiving a data frame. Inserted into the ACK are a listening period, and a phase (CSLPhase) indicating a gap between the time of synchronization information in the wakeup frame and the reception time.
If the transmission device S receives no ACK from the reception device R, the transmission device S transmits a wakeup frame and a data frame to the reception device R in the asynchronous transmission mode again in transmitting the next data frame.
If the transmission device S receives an ACK from the reception device R, the transmission device S enters a synchronous transmission mode. The transmission device S retains a phase and corrects synchronization with the reception device R in the listening period in the ACK. The transmission device S then transmits a wakeup frame and a data frame at the corrected transmission timing in transmitting the next data frame. Wakeup frame transmission time (or the number of transmission) in the synchronous transmission mode may only be time for which synchronization can be corrected. Compared with the asynchronous transmission mode, this transmission time (or the number of transmission) can be remarkably reduced to from 10% to 1%.
If a transmission device other than the transmission device S performs wireless transmission, the transmission interferes with the transmission of the transmission device S and the reception device R becomes unable to receive a wakeup frame. Methods for avoiding such a collision include carrier sense multiple access with collision avoidance (CSMA/CA).
For example, the CSMA/CA described in IEEE802.15.4 is the following algorithm. FIG. 11 is a flowchart illustrating the CSMA/CA algorithm.
The transmission device S sets, for example, at 5 in advance, the minimum value (macMinBE) of a backoff exponent (which will also be referred to as “BE” below) in control information related to communication control which is referred to as personal area network information base (which will also be referred to as “PIB” below). The backoff exponent (BE) is a parameter used for a function random discussed below.
In addition, the transmission device S sets the maximum number of backoff (macMaxCSMABackoff), for example, at 4 in advance. The NB is a counter that will be discussed below and counts the number of CCA failure. Meanwhile, the random is a function that outputs a random value for an input value.
If there is a frame transmission request, the transmission device S sets 0 for the NB and sets the value of the macMinBE for the BE in the first place (S001).
The transmission device S gives as much delay as time obtained by multiplying a unit backoff period by an output result of a random (2BE-1) (S002).
The transmission device S performs clear channel assessment (which will also be referred to as “CCA” below) and examines whether or not a corresponding CH is in use (S003).
If the corresponding CH is idle or no wireless carrier is sensed, the transmission device S can transmit a frame as a success (S004).
If the transmission device S senses a wireless carrier, the transmission device S increases the NB by one. Meanwhile, the transmission device S also increases the BE by one (S005).
If the NB exceeds the maximum number of backoff, the transmission device S, as failure, does not perform transmission (S006).
If the NB does not exceed the maximum number of backoff, the transmission device S returns to the process in step S002 again and executes the processes in the following steps. The value of the BE increases if the transmission device S senses a wireless carrier. Accordingly, there is a high probability that a delay amount in one period increases.
There may only be time for which the reception device R can receive any of transmitted wakeup frames as active time for which the reception device R performs wakeup reception. Shorter active time brings about more power-saving effects, but too short active time results in difficulty in receiving a wakeup frame.
FIG. 12 is an explanatory diagram illustrating that a plurality of transmission devices make transmission requests at the same time.
A transmission device S1 transmits data to the reception device R in a CSL mode. A transmission device S2 transmits data to a communication device other than the transmission device S1 and the reception device R in a normal mode (a communication mode other than the CSL mode is referred to as “normal mode”).
Let us assume that the transmission device S1 attempts to transmit a wakeup frame 21 and the transmission device S2 attempts to transmit a data frame 11 at time t11.
Since delay time caused by backoff, which is computed in the process of step S002 discussed above, is multiplied by a random value, it is not know which of the transmission device S1 and the transmission device S2 has longer delay time. If the transmission device S2 has shorter delay time caused by backoff, the transmission device S1 performs wireless carrier sensing on the transmission of the transmission device S2 while the transmission device S1 is performing CCA. Consequently, after the transmission device S2 completes transmitting the data frame 11, the transmission device S1 transmits the wakeup frame 21.
If the transmission timing of a wakeup frame 22 from the transmission device S1 is delayed, the reception device R fails to receive the wakeup frame 22 for active time 33.
In this way, even if the reception device R is correctly synchronized with the transmission device S1, the CSMA/CA causes transmission delay of a wakeup frame or causes transmission intervals to be longer and the reception device R fails to receive the wakeup frame within the active time in some cases. If the reception device R does not receive any wakeup frame, the reception device R does not reach the listening state in receiving data. Consequently, a reception error occurs with respect to a data frame.